The present invention relates to a telecommunications system comprising at least one primary radio station and a plurality of secondary radio stations, in which at least one secondary radio station is matched to a primary radio station, wherein the stations have a transmitter and a receiver. Such a telecommunications system can be a cordless telecommunications system in which radio stations are matched to each other via an identification code, or any other suitable system in which matched radio stations communicate with each other.
A telecommunications system of the above kind is a known CT0 or CT1 telephone readily available onto the market. When intended for the European market, such phones operate with paired channels for a full duplex communication link, and when intended for the US market, such phones at least partly operate with unpaired channels. At call establishment, channels are scanned using a channel selection protocol. No busy channels may be applied to a new call. To this end the field strength of the received signal is measured and a channel is considered free if the field strength is below a given threshold. Cordless phones are sold comprising a base station coupled to the public switched telephone network and at least one portable station having an identification code matching the base station""s identification code. When having a so-called multi-handset cordless phone, several additional functions can be implemented whereby the base station has information for all handsets. The base station could broadcast a ringing signal to all handsets, for instance.
It is an object of the present invention to provide a telecommunications system of the above, kind with an efficient and reliable point to multipoint radio protocol.
Accordingly to the present invention is characterised in that the primary station is arranged for setting up a point to point full duplex communication link to a matched secondary station or vice versa, and that the primary station is arranged for issuing an extension request to the matched secondary station and to further matched secondary stations after the setting up of the point to point communication link, and that the secondary station to which the communication link has been set up switches off its transmitter, and that the matched secondary station and the further matched secondary stations have their receivers ready for receiving a broadcast message from the primary station. The present invention is based upon the insight that by first establishing a reliable point to point duplex link to a handset or by starting from a reliable existing radio link, and then commanding the one handset to switch off its transmitter. By transmitting the extension message continuously, handsets not having received earlier extension messages can join the extension process. Such a situation could occur if an additional handset is switched on or comes out of a radio shadow.
In an embodiment of a telecommunications system according to the present invention, the primary station is arranged for issuing a release request to matched secondary stations receiving broadcast messages from the primary station after having received the extension request, the secondary stations releasing the communication link with the primary station after having received the release request. Herewith, the broadcast channel comes available again.
In a different embodiment of a telecommunications system according to the present invention the primary station is arranged for setting up a point to point full duplex communication link to a matched secondary station or vice versa, that the secondary station to which the full duplex communication has been set up is arranged for transmitting a transfer message to the primary station for transferring the link to another matched secondary station. The primary station transmits a release request to the secondary station to which the link has been set up, and that the primary station issues a link establishment request for establishing a full duplex communication link with the other matched secondary station. Herewith, a set up link can reliably be transferred to another station.
In a still different embodiment of a telecommunications system according to the present invention, the primary station is arranged for setting up a point to point full duplex communication link to a matched secondary station or vice versa. The secondary station to which the full duplex communication has been set up is arranged for transmitting a channel skip message to the primary station for skipping the link to another channel. The primary station transmits a release request to the secondary station to which the link has been set up in response to which the secondary station enters a channel scanning mode that the primary station issues a link establishment skip request for establishing another full duplex communication link with the secondary station. The primary station then transmits a skip off message to the secondary station after having set up the other communication link. Herewith, reliable channel skipping is achieved.
In an embodiment of a telecommunications system according to the present invention, the station establishing the point to point communication link is arranged for issuing an establishment request for establishing the communication link with the opposite station while applying a double checking protocol for checking whether both channels in the duplex communication link are free, which double checking protocol comprises the following steps:
a) after having found a free channel, the establishing station switches on its transmitter and transmits at least one activation message to the opposite station,
b) after having received and decoded an activation message, the opposite station switches on its transmitter on a free channel and checks for a first predetermined period of time whether the establishing station has switched off its transmitter within the first period for channel on which its transmits the at least one activation message,
c) the establishing station switches off its transmitter if a busy channel of the opposite station is found,
d) if the channel is free, the opposite station transmits at least one acknowledgement message on the channel it has selected after having received the activation message, and
e) the establishing station switches on its transmitter on the channel it was transmitting the at least one activation message after having received and decoded the acknowledgement message. Herewith, full duplex link establishments can be done very reliable, both for systems with unpaired and for systems with paired duplex channels. Because such point to point link establishment is also used in the multi-handset broadcast protocol, this protocol is also very reliable. The double checking protocol itself is claimed and described in a patent application (copending application Ser. No. 08/976,647) filed by the same applicant on the same day.